make sure your running the newest bios file on the motherboard. if your using an asus mb make sure it running in standard mode. in performance mode the ram to cpu timing can be off. use a volt meter and watch your power supply voltages see if they drop below atx spec. if the voltages are holding if you have a electronic school or a tv repair store that has an o-scope have them check the power supply for ac ripple. pc need clean dc power if an ac ripple gets into a mb it can cause it to lock up or reset. the last issues can be bad cache in a cpu or bad hyper threading in the cpu if you have an i7 turn off hyper threading in the bios. the last issues and hardest to find would be a bad cap or vrm on your mother board.